Bright white flashes when eyes closed?!

A couple of year ago I went to go to sleep as you do, my eyes were closed and my room pitch black, no lights outside none in the hall complete darkness. As I was drifting off a bright flash went infront of my eyes. It was a yellowy white and was as if someone had lit a lighter infront of my face. It flashed a couple of times and went. I've experienced it a few times since but its since been a brilliant white like and a lot bigger more like a torch or flash bulb on a camera but not as instant As a camera flash it lingers for a second or 2. I'm not on any meds, drugs, drink. Nothing traumatic or stressing, normal sleep patterns. What could this bright white flash be? Thanks all.